1. Field of the Invention
The present invention relates to pressure to electrical signal transmitters. More specifically, the present invention is directed to an overload protection mechanism for a differential pressure transmitter.
2. Description of the Prior Art
Differential pressure transmitters for producing an electrical signal of a value dependent upon the difference between two fluid pressures are well-known in the art. This type of transmitter usually employs a barrier diaphragm between an input process fluid and a fill fluid contained within the transmitter body. The barrier diaphragm transmits the pressure of the input fluid to the fill fluid which, in turn, transmits fluid pressure to a pressure sensor. An example of such a transmitter employing a pressure sensor which has only a relatively small displacement over its operating range is shown in the Whitehead, Jr. U.S. Pat. No. 4,072,058. In this transmitter, the pressure sensor is a semiconductor strain-sensitive wafer. Differential pressure transducers of the form shown in the aforesaid patent include a so-called head, or transmitter body, structure which contains two barrier diaphragms. A chamber on one side of one barrier diaphragm is pressurized with a first input fluid having a first pressure and a chamber on one side of the other barrier diaphragm is pressurized with a second input fluid having a second pressure. These two input fluids may be so-called process fluids which can be obtained from process flow lines, e.g., from respective sides of an orifice plate. A separate chamber on the other side of each of the barrier diaphragms contains a corresponding fill fluid. These fill fluids communicate through fluid passages with opposite sides of the pressure sensor, and the latter is displaced by an amount which is dependent upon the difference between the two pressures of the fill fluids, i.e., upon the differential pressure of the two applied input fluids. The sensor produces an electrical output signal which is dependent upon the displacement and, hence, upon the differential pressure.
Because of the minute full range displacement of the sensor in such transmitters, it has been necessary to provide such transmitters with overload protection arrangements to protect the sensor from being excessively displaced and, hence, either damaged or destroyed by excessive applied input fluid pressures. In the aforesaid U.S. patent, such an overload protection is provided for each barrier diaphragm by a corresponding circular spring which is arranged to preload a respective control diaphragm. The control diaphragms are each arranged to valve or divert a respective fill fluid into an expansion chamber to relieve the excess pressure on the sensor. An applied pressure which is in excess of the preload pressure is effective to move the control diaphragm whereby the excessive input pressure is diverted from the sensor. Ultimately, the diversion of the fill fluid allows the barrier diaphragm to bottom onto a back-up plate to prevent any further input pressure increase from being applied to the sensor. However, it has been found that in order to change the working range of the pressure transmitter, the springs used to preload the control diaphragms must be changed to produce a new working range for the overload protection. Such characterization of the preload springs involves a disassembly and reassembly of the transmitter structure which is an inefficient and expensive operation.